LithosAnanake/src/starkernel/vm/switch.c

/*
  StarForth — Steady-State Virtual Machine Runtime

  Copyright (c) 2023–2025 Robert A. James
  All rights reserved.

  This file is part of the StarForth project.

  Licensed under the StarForth License, Version 1.0 (the "License");
  you may not use this file except in compliance with the License.

  You may obtain a copy of the License at:
      https://github.com/star.4th@proton.me/StarForth/LICENSE.txt

  This software is provided "AS IS", WITHOUT WARRANTY OF ANY KIND,
  express or implied, including but not limited to the warranties of
  merchantability, fitness for a particular purpose, and noninfringement.

 See the License for the specific language governing permissions and
 limitations under the License.
*/

/**
 * switch.c - Cooperative VM context switch glue (FABRIC-3.md §XXVIII,
 * Stage 2, 2026-09-13). See switch.h for the contract.
 */

#ifdef __STARKERNEL__

#include "starkernel/vm/switch.h"
#include "vm.h"
#include "starkernel/capsule_birth.h" /* capsule_vm_set_state */
#include "starkernel/vm/kernel_hermes.h" /* FABRIC-3.8.md §3.2 -- sk_hermes_drain_checkpoint() */
#include "starkernel/hal/hal.h" /* sk_hal_panic() */
#include "starkernel/capsule_vm_switch_signal.h" /* FABRIC-3.8.md §3.13.3 -- the decision */
#include "starkernel/vm/bootstrap/sk_vm_bootstrap.h" /* sk_get_mama_vm() -- the idle root */
#include <stdint.h>

/* Arch-specific (switch.S in each arch dir). sk_vm_switch_prime() builds
 * the exact synthetic first-entry frame sk_vm_switch_to() expects -- kept
 * in assembly, not duplicated here, so the layout can never drift out of
 * sync between the two. */
extern void     sk_vm_switch_to(uint64_t *save_sp_out, uint64_t new_sp);
extern uint64_t sk_vm_switch_prime(uint64_t stack_top, void *entry);

/* Single-writer-mainline global, safe under this stage's cooperative-only,
 * one-VM-runs-at-a-time-on-one-hart discipline (same class of bookkeeping
 * as vm_core.c's g_log_attrib_vm) -- identifies which VM a freshly-primed
 * native stack belongs to. Read exactly once, synchronously, at the very
 * top of sk_vm_switch_entry() before any other first-entry could occur.
 * Safe to keep as a plain global (unlike the old g_switch_back_to this
 * file used to also carry -- see native_switch_back_to's own doc comment
 * in vm.h for why that one had to move to per-VM state): a VM's identity
 * never changes across the lifetime of its own trampoline instance, only
 * who most recently switched into it does. */
static VM *g_switch_entry_vm;

/* FABRIC-3.md §XXVIII Stage 3 follow-on (2026-09-14): which VM is
 * currently running, from the switch mechanism's own point of view --
 * independent of vm_log_attributed_vm() (vm_core.c), which is scoped to
 * nested vm_interpret() calls for log-source attribution and has no way
 * to know a VM is "running" while parked inside this file's own raw C
 * trampoline (sk_vm_switch_entry()), never itself calling vm_interpret().
 * sk_vm_switch_signal_tick() (capsule_vm_switch_signal.c) consults this
 * instead of vm_log_attributed_vm() for exactly that reason -- see its
 * own updated comment. Single-writer-mainline, same discipline as
 * g_switch_entry_vm above: set unconditionally on every switch, by the
 * only code path that can ever change who is physically running.
 * Seeded once at boot via sk_vm_switch_set_current() for whichever VM is
 * running before any switch has ever happened (Hera, at Stage 3
 * registration time -- kernel_main.c). */
static VM *g_switch_current_vm;

VM *sk_vm_switch_current_vm(void) { return g_switch_current_vm; }

void sk_vm_switch_set_current(VM *vm) { g_switch_current_vm = vm; }

/**
 * sk_vm_switch_entry - trampoline for a VM's first-ever entry.
 *
 * A freshly-entered VM has no production behavior defined yet -- what a
 * switched-to VM should actually DO once running is later-stage work
 * (message dispatch, an idle loop, whatever Stage 3+ needs). For now this
 * is a minimal, permanent placeholder: immediately yield back to whoever
 * most recently switched to it, forever. Reads self->native_switch_back_to
 * fresh every iteration (not a value captured once) so a later switch-in
 * from a different VM is honored, not silently bounced to whoever entered
 * first. Must never fall off the end -- there is no legitimate return
 * address below a synthetic first-entry frame, unlike an ordinary
 * function.
 *
 * FABRIC-3.md §XXVIII Stage 3 follow-on (2026-09-14): an attempt to make
 * this attributable by calling vm_interpret(self, "MSG-TICK") here was
 * tried and reverted -- it introduced a second, distinct hang (a stall in
 * sk_repl_headless_wait()'s WIREBIND-attachment wait, not root-caused) in
 * place of the switch-storm it was meant to fix. The switch-storm's real
 * cause (sk_vm_switch_signal_tick()'s "who is current" check via
 * vm_log_attributed_vm() never seeing a VM that only ever runs this raw
 * C trampoline) is instead fixed at the source in switch.c/
 * capsule_vm_switch_signal.c: sk_vm_context_switch() now tracks "current"
 * itself, independent of vm_interpret()'s own attribution bookkeeping.
 * This placeholder's behavior is therefore unchanged from Stage 2.
 */
/*
 * FABRIC-3.8.md §3.2 (Stage B, 2026-09-23): the placeholder above is
 * superseded -- this trampoline is now every switch participant's own
 * kernel-side idle loop, the base of its native stack. Entirely OS-driven:
 * nothing a VM executes decides to give up control (Captain Bob: "no
 * special words saying I'm giving up control... strictly all operating
 * system and strictly all compudynamically driven"). Each pass:
 *
 *   - Drain one of this VM's own kernel-Hermes messages, in its own
 *     context, on its own stack (sk_hermes_drain_checkpoint() at depth 0
 *     here, so the payload runs at this VM's depth 1 and its own word
 *     checkpoints pass the Stage A ownership gate).
 *   - If nothing was drained, this VM has no work: mark it parked-idle
 *     (§3.5 -- nothing borrowed on its stack, nothing in progress) and
 *     yield back to whoever switched in.
 *
 * Driven off the drain's return value, never a separate pending-count
 * probe: any path where a drain returns without consuming a message would
 * otherwise spin here forever with no word checkpoint to break it --
 * indistinguishable from a switch storm. A failed yield is a kernel
 * invariant violation, not something to retry, so it panics loudly.
 *
 * Stage C-b (below) adds the compudynamic decision at this point between
 * messages, so a VM with a queue of checkpoint-free payloads cannot hold
 * the CPU indefinitely.
 */
/*
 * FABRIC-3.8.md §3.13.3 (Stage C-b, 2026-09-23): the trampoline now asks
 * the compudynamic decision at its between-messages point, instead of
 * always draining to empty and then yielding to whoever switched in:
 *
 *   - still has mail: keep draining unless some VM with work is strictly
 *     cooler (then switch to it; this VM stays eligible through its mail);
 *   - no mail: switch to the coolest VM with work, or -- nobody has any --
 *     to Hera, the console root, whose idle point polls for input.
 *
 * The no-work fallback is Hera rather than native_switch_back_to because
 * back_to is "whoever switched in last": two idle trampolines that switched
 * into each other would each yield to the other forever -- a switch storm.
 * Hera is never waiting on a trampoline, so the chain always ends at her.
 */
void sk_vm_switch_entry(void) {
    VM *self = g_switch_entry_vm;
    for (;;) {
        VM *next;
        int drained, has_mail;

        drained  = sk_hermes_drain_checkpoint(self); /* at most one message */
        has_mail = sk_hermes_pending_count(self->stadium_vm_id) > 0;
        /* Stage B's rule, kept: never spin on mail the drain won't take --
         * with no word checkpoint in this loop, that would be a silent storm. */
        if (drained == 0 && has_mail) {
            sk_hal_panic("switch trampoline: pending mail the drain did not consume");
        }
        sk_vm_switch_reap_pending();
        next = sk_vm_switch_decide(self, has_mail);
        if (!next) {
            if (has_mail) continue;          /* still coolest with work: keep going */
            next = (VM *)sk_get_mama_vm();   /* nobody has work: back to the root */
            if (!next || next == self) sk_hal_panic("switch trampoline: no root to idle to");
        }

        /* Stage FR (§3.12): parked is a VM_FRAME_IDLE frame on this VM's own
         * stack, pushed only around the switch and popped (by ID) on resume,
         * BEFORE any further drain -- so the drain's own frame is still the
         * only frame and the ownership gate's exactly-one-frame rule holds.
         * It records WHERE the VM is parked: at its trampoline, nothing in
         * progress (any remaining mail keeps it eligible by itself). */
        {
            VMFrameID idle_id = vm_frame_push(&self->frames, self->stadium_vm_id, VM_FRAME_IDLE);
            if (idle_id == 0) {
                sk_hal_panic("switch trampoline: idle frame push failed");
            }
            sk_vm_switch_signal_note_switch_performed(next->stadium_vm_id);
            if (sk_vm_context_switch(self, next) != 0) {
                sk_hal_panic("switch trampoline: switch failed");
            }
            vm_frame_pop(&self->frames, idle_id); /* resumed: someone switched back in */
        }
    }
}

/* See switch.h. The caller has already established that `vm` owns its
 * outermost frame (sk_vm_owns_outermost_frame(), vm_core.c). */
void sk_vm_switch_checkpoint(VM *vm) {
    VM *next;

    if (!sk_vm_switch_signal_take_decision_due()) return;
    sk_vm_switch_reap_pending();
    next = sk_vm_switch_decide(vm, 1); /* vm is mid-task: it still has work */
    if (!next) return;
    sk_vm_switch_signal_note_switch_performed(next->stadium_vm_id);
    (void)sk_vm_context_switch(vm, next);
    /* Resumes here once the decision, somewhere, switches back to vm. */
}

/* See switch.h. FABRIC-3.8.md §3.13.5 (ruling C-5): Hera between lines,
 * polling for input, is the idle point. She has no work of her own there,
 * so any VM with work goes -- the coolest one. She is parked with an IDLE
 * frame (nothing in progress), so she becomes eligible again only through
 * mail or pending console input, or is returned to by a trampoline with
 * nothing left to do. */
void sk_vm_idle_point(void) {
    VM *self = g_switch_current_vm;
    VM *next;

    if (!self || self != (VM *)sk_get_mama_vm()) return;
    if (self->frames.count != 0) return; /* inside a word (KEY/EXPECT): not idle */

    sk_vm_switch_reap_pending();
    next = sk_vm_switch_decide(self, 0);
    if (!next) return;
    {
        VMFrameID idle_id = vm_frame_push(&self->frames, self->stadium_vm_id, VM_FRAME_IDLE);
        if (idle_id == 0) return; /* fail closed: stay put rather than park unmarked */
        sk_vm_switch_signal_note_switch_performed(next->stadium_vm_id);
        (void)sk_vm_context_switch(self, next);
        vm_frame_pop(&self->frames, idle_id);
    }
}

int sk_vm_context_switch(VM *from, VM *to) {
    uint64_t new_sp;

    if (!from || !to || to->native_stack_top == 0) {
        return -1;
    }

    /* Recorded unconditionally (first-ever entry or a resume) so whoever
     * is parked at `to`'s native stack -- most often this placeholder
     * trampoline -- always knows who most recently switched into it. */
    to->native_switch_back_to = from;

    if (to->native_stack_saved_sp == 0) {
        /* First-ever entry: synthesize the initial frame. */
        g_switch_entry_vm = to;
        new_sp = sk_vm_switch_prime(to->native_stack_top, (void *)&sk_vm_switch_entry);
    } else {
        new_sp = to->native_stack_saved_sp;
        to->native_stack_saved_sp = 0; /* about to be running, not parked */
    }

    capsule_vm_set_state(to->stadium_vm_id, VM_STATE_LIVE);
    capsule_vm_set_state(from->stadium_vm_id, VM_STATE_SWITCHED_OUT);
    sk_vm_switch_signal_note_parked(from->stadium_vm_id); /* §3.13.3 wait stamp */
    g_switch_current_vm = to;

    sk_vm_switch_to(&from->native_stack_saved_sp, new_sp);

    /* Resumes here only once something later switches back to `from` --
     * from `from`'s own point of view, this call simply took a while. */
    capsule_vm_set_state(from->stadium_vm_id, VM_STATE_LIVE);
    g_switch_current_vm = from;
    return 0;
}

#endif /* __STARKERNEL__ */